1. Field of the Invention
The present invention generally relates to a double helical gear (or a herringbone gear) formed from a resin, and a gear train using the same. More specifically, the invention relates to a resin double helical gear widely used for various image forming systems, such as copying machines, facsimile terminal equipment and printers, various automotive parts, such as wiper driving units, power window driving units and auto slide door driving units, various electronic equipment, and precision instruments, which are required to silently transmit power at a high rotational speed and at a high load and which are required to be lightened and inexpensively produced.
2. Description of the Prior Art
A typical ink jet printer is often connected to a personal computer to be used, and is arranged on or near a desk, on which the personal computer is operated. It is desirable to suppress the operation noises and vibrations of the ink jet printer.
Therefore, in conventional ink jet printers, a helical gear is used as each of an output gear of a motor and an idle gear meshing with the output gear, and the contact ratio of the output gear with the idle gear is increased to decrease noises. In addition, a spur gear is used as each of a driven gear of a roller shaft and an idle gear meshing therewith to prevent a thrust force from being applied to the roller shaft. Furthermore, if a thrust force is applied to the roller shaft, a transporting roller moves so as to be dislocated in the axial direction of the roller shaft. Then, a sheet (a sheet-like recording material, such as a copy paper or a post card), transported by the transporting roller, is transported so as to be dislocated in the axial directions of the roller shaft. Thus, the position at which printing starts is deviated so that the precision of printing is deteriorated.
However, if the helical gears are used as the output gear and the idle gear meshing therewith, like in conventional ink jet printers, the thrust force is applied to both of the gears. Thus there are some cases where vibrations are caused by the backlash of both of the gears and the mounting portion of the rotational shaft rotatably supporting thereon the gears.
Resin helical gears can smoothly mesh with another gear at a high rotational speed, and can silently transmit power. In addition, resin helical gears allow a load to be easily dispersed along the tooth trace, and can transmit power at a high load. Therefore, conventionally, resin helical gears have been widely used for the power transmission devices of business machines, such as copying machines, printers and facsimile terminal equipments, as well as power transmission devices of automotive parts, precision instruments and various electronic equipment.
However, in resin helical gears, there is a disadvantage in that an axial thrust load directly proportional to torque is caused since the teeth are helical. In order to eliminate such a disadvantage of resin helical gears, the thrust load caused during the transmission of power must be received by a thrust bearing. However, there are some cases where the thrust bearing can not be arranged due to constraints on space and the configuration of power transmission devices.
There has been developed a technique for forming a resin double helical gear, which can silently transmit power at a high speed and at a high load similar to a resin helical gear and which can prevent a thrust load from being caused during the transmission of power, by injection molding (see Japanese Patent Laid-Open No. 10-315344).
As shown in FIG. 23, a resin double helical gear 130 disclosed in Japanese Patent Laid-Open No. 10-315344 is formed in large quantities in a short time by the steps of: causing a first die 133, which is designed to form a portion (a first gear portion 132) on one side in face width directions from a central portion 131 in the face width directions, to butt a second die 135, which is designed to form a portion (a second gear portion 134) on the other side in the face width directions from the central portion 131; injecting a molten resin into a cavity 136 defined by the first die 133 and the second die 135; separating the first die 133 and the second die 135 from each other after the resin in the cavity 136 is cooled and solidified; and removing the solidified resin from the cavity 136 defined by the first die 133 and second die 135.
When the separated dies (the first die 133 and the second die 135) are caused to butt each other to allow injection of the molten resin into the cavity 136 to mold the resin double helical gear 130, even if a butt surface 137 between both of the dies 133 and 135 is slightly dislocated, the butt surface 137 between the first gear portion 132 and second gear portion 134 of the injection molded resin double helical gear 130 is also dislocated as shown in FIGS. 12A and 12B. As a result, there is a problem in that a stepped portion (dislocation) 117 is produced at the top portion 104 of a chevron-shaped tooth 138 and on the reverse 116 thereof. If such a stepped portion 117 is produced at the top portion 104 of the chevron-shaped tooth 138 and on the reverse 116 thereof, only a half tooth portion (the first gear portion 132 or the second gear portion 134) in face width directions transmits power, so that thrust is produced similar to helical gears.
In such a case, it is considered that a slit 140 extending along the butt surface 137 between the first gear portion 132 and the second gear portion 134, as shown in FIG. 24 is formed to prevent a discrepant meshing due to the interference with the stepped portion 117 at the top portion 104 of each tooth of a companion resin double helical gear 130 meshing therewith or on the reverse 116 thereof. However, there are problems in that the structure of the die is complicated and the effective facewidth relating to the flexural strength of the tooth is decreased.